linux/Documentation/vgaarbiter.txt
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   2VGA Arbiter
   3===========
   4
   5Graphic devices are accessed through ranges in I/O or memory space. While most
   6modern devices allow relocation of such ranges, some "Legacy" VGA devices
   7implemented on PCI will typically have the same "hard-decoded" addresses as
   8they did on ISA. For more details see "PCI Bus Binding to IEEE Std 1275-1994
   9Standard for Boot (Initialization Configuration) Firmware Revision 2.1"
  10Section 7, Legacy Devices.
  11
  12The Resource Access Control (RAC) module inside the X server [0] existed for
  13the legacy VGA arbitration task (besides other bus management tasks) when more
  14than one legacy device co-exists on the same machine. But the problem happens
  15when these devices are trying to be accessed by different userspace clients
  16(e.g. two server in parallel). Their address assignments conflict. Moreover,
  17ideally, being an userspace application, it is not the role of the the X
  18server to control bus resources. Therefore an arbitration scheme outside of
  19the X server is needed to control the sharing of these resources. This
  20document introduces the operation of the VGA arbiter implemented for Linux
  21kernel.
  22
  23----------------------------------------------------------------------------
  24
  25I.  Details and Theory of Operation
  26        I.1 vgaarb
  27        I.2 libpciaccess
  28        I.3 xf86VGAArbiter (X server implementation)
  29II. Credits
  30III.References
  31
  32
  33I. Details and Theory of Operation
  34==================================
  35
  36I.1 vgaarb
  37----------
  38
  39The vgaarb is a module of the Linux Kernel. When it is initially loaded, it
  40scans all PCI devices and adds the VGA ones inside the arbitration. The
  41arbiter then enables/disables the decoding on different devices of the VGA
  42legacy instructions. Device which do not want/need to use the arbiter may
  43explicitly tell it by calling vga_set_legacy_decoding().
  44
  45The kernel exports a char device interface (/dev/vga_arbiter) to the clients,
  46which has the following semantics:
  47
  48 open       : open user instance of the arbiter. By default, it's attached to
  49              the default VGA device of the system.
  50
  51 close      : close user instance. Release locks made by the user
  52
  53 read       : return a string indicating the status of the target like:
  54
  55              "<card_ID>,decodes=<io_state>,owns=<io_state>,locks=<io_state> (ic,mc)"
  56
  57              An IO state string is of the form {io,mem,io+mem,none}, mc and
  58              ic are respectively mem and io lock counts (for debugging/
  59              diagnostic only). "decodes" indicate what the card currently
  60              decodes, "owns" indicates what is currently enabled on it, and
  61              "locks" indicates what is locked by this card. If the card is
  62              unplugged, we get "invalid" then for card_ID and an -ENODEV
  63              error is returned for any command until a new card is targeted.
  64
  65
  66 write       : write a command to the arbiter. List of commands:
  67
  68  target <card_ID>   : switch target to card <card_ID> (see below)
  69  lock <io_state>    : acquires locks on target ("none" is an invalid io_state)
  70  trylock <io_state> : non-blocking acquire locks on target (returns EBUSY if
  71                       unsuccessful)
  72  unlock <io_state>  : release locks on target
  73  unlock all         : release all locks on target held by this user (not
  74                       implemented yet)
  75  decodes <io_state> : set the legacy decoding attributes for the card
  76
  77  poll               : event if something changes on any card (not just the
  78                       target)
  79
  80  card_ID is of the form "PCI:domain:bus:dev.fn". It can be set to "default"
  81  to go back to the system default card (TODO: not implemented yet). Currently,
  82  only PCI is supported as a prefix, but the userland API may support other bus
  83  types in the future, even if the current kernel implementation doesn't.
  84
  85Note about locks:
  86
  87The driver keeps track of which user has which locks on which card. It
  88supports stacking, like the kernel one. This complexifies the implementation
  89a bit, but makes the arbiter more tolerant to user space problems and able
  90to properly cleanup in all cases when a process dies.
  91Currently, a max of 16 cards can have locks simultaneously issued from
  92user space for a given user (file descriptor instance) of the arbiter.
  93
  94In the case of devices hot-{un,}plugged, there is a hook - pci_notify() - to
  95notify them being added/removed in the system and automatically added/removed
  96in the arbiter.
  97
  98There's also a in-kernel API of the arbiter in the case of DRM, vgacon and
  99others which may use the arbiter.
 100
 101
 102I.2 libpciaccess
 103----------------
 104
 105To use the vga arbiter char device it was implemented an API inside the
 106libpciaccess library. One field was added to struct pci_device (each device
 107on the system):
 108
 109    /* the type of resource decoded by the device */
 110    int vgaarb_rsrc;
 111
 112Besides it, in pci_system were added:
 113
 114    int vgaarb_fd;
 115    int vga_count;
 116    struct pci_device *vga_target;
 117    struct pci_device *vga_default_dev;
 118
 119
 120The vga_count is usually need to keep informed how many cards are being
 121arbitrated, so for instance if there's only one then it can totally escape the
 122scheme.
 123
 124
 125These functions below acquire VGA resources for the given card and mark those
 126resources as locked. If the resources requested are "normal" (and not legacy)
 127resources, the arbiter will first check whether the card is doing legacy
 128decoding for that type of resource. If yes, the lock is "converted" into a
 129legacy resource lock. The arbiter will first look for all VGA cards that
 130might conflict and disable their IOs and/or Memory access, including VGA
 131forwarding on P2P bridges if necessary, so that the requested resources can
 132be used. Then, the card is marked as locking these resources and the IO and/or
 133Memory access is enabled on the card (including VGA forwarding on parent
 134P2P bridges if any). In the case of vga_arb_lock(), the function will block
 135if some conflicting card is already locking one of the required resources (or
 136any resource on a different bus segment, since P2P bridges don't differentiate
 137VGA memory and IO afaik). If the card already owns the resources, the function
 138succeeds.  vga_arb_trylock() will return (-EBUSY) instead of blocking. Nested
 139calls are supported (a per-resource counter is maintained).
 140
 141
 142Set the target device of this client.
 143    int  pci_device_vgaarb_set_target   (struct pci_device *dev);
 144
 145
 146For instance, in x86 if two devices on the same bus want to lock different
 147resources, both will succeed (lock). If devices are in different buses and
 148trying to lock different resources, only the first who tried succeeds.
 149    int  pci_device_vgaarb_lock         (void);
 150    int  pci_device_vgaarb_trylock      (void);
 151
 152Unlock resources of device.
 153    int  pci_device_vgaarb_unlock       (void);
 154
 155Indicates to the arbiter if the card decodes legacy VGA IOs, legacy VGA
 156Memory, both, or none. All cards default to both, the card driver (fbdev for
 157example) should tell the arbiter if it has disabled legacy decoding, so the
 158card can be left out of the arbitration process (and can be safe to take
 159interrupts at any time.
 160    int  pci_device_vgaarb_decodes      (int new_vgaarb_rsrc);
 161
 162Connects to the arbiter device, allocates the struct
 163    int  pci_device_vgaarb_init         (void);
 164
 165Close the connection
 166    void pci_device_vgaarb_fini         (void);
 167
 168
 169I.3 xf86VGAArbiter (X server implementation)
 170--------------------------------------------
 171
 172(TODO)
 173
 174X server basically wraps all the functions that touch VGA registers somehow.
 175
 176
 177II. Credits
 178===========
 179
 180Benjamin Herrenschmidt (IBM?) started this work when he discussed such design
 181with the Xorg community in 2005 [1, 2]. In the end of 2007, Paulo Zanoni and
 182Tiago Vignatti (both of C3SL/Federal University of Paran\xE1) proceeded his work
 183enhancing the kernel code to adapt as a kernel module and also did the
 184implementation of the user space side [3]. Now (2009) Tiago Vignatti and Dave
 185Airlie finally put this work in shape and queued to Jesse Barnes' PCI tree.
 186
 187
 188III. References
 189==============
 190
 191[0] http://cgit.freedesktop.org/xorg/xserver/commit/?id=4b42448a2388d40f257774fbffdccaea87bd0347
 192[1] http://lists.freedesktop.org/archives/xorg/2005-March/006663.html
 193[2] http://lists.freedesktop.org/archives/xorg/2005-March/006745.html
 194[3] http://lists.freedesktop.org/archives/xorg/2007-October/029507.html
 195
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